Water quality monitoring strategies and their effects on mass load calculation
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Simulation methods are needed to aid in the design of watershed water quality sampling strategies and the calculation of mass loads for watershed monitoring programs. A simulation tool to help determine the influence of water quality monitoring strategies on the estimation of mass load for constituent responses to watershed discharge was developed. For this work, four chemograph profiles were constructed and simulated for 100 simulated runoff events at three different temporal and spatial scales. Seven sampling strategies were employed at various frequencies to determine mass loads for each event, including time-based fixed-frequency, composite, and flow-proportional sampling strategies. The limits of tested frequencies were from 5 to 360 min and 2 to 15,000 m 3 sampling for fixed-frequency and flow-proportional strategies, respectively, with additional frequencies analyzed, including compositing of aliquots of samples. Total and propagated mass load errors were calculated. Error ranged from less than 0.01 to more than 0.75 (unitless proportional values) compared to the simulated values, the mass load value calculated from the full unsampled data set, and was influenced primarily by the sampling frequency, chemograph profile, and sampling strategy. Each tested strategy introduced error in different ways, thereby requiring that recommendations be site and objective specific; however, flow-proportional strategies were the most accurate and sample-intensive, with error ranging from 0.01 to 0.30. For situations requiring few samples, composite strategies were the most accurate, with error ranging from 0.05 to 0.34. The effects of spatial scale, estimated by using watershed areas of 100 ha, 1,000 ha, and 10,000 ha, and chemograph randomness were also examined, with results indicating that error also increased by orders of magnitude with increasing watershed size. 2007 American Society of Agricultural and Biological Engineers.
